Fat Digestion in ruminants
Fate of rumen lipids
Dietary lipids occur as structural lipids in forages and storage lipids in oilseeds. In forages, less than 50% of total lipids are free fatty acids and the majority of them are phospholipids.
In oilseeds, 65-80% of lipids are free fatty acids.
Rumen microbes hydrolyze the dietary lipids to glycerol and free fatty acids. Glycerol is converted to propionic acid by the microbes.
- The metabolism of lipids by microorganisms takes place in 4 steps:
- Hydrolysis of esterified fatty acids.
- Biohydrogenation of unsaturated fatty acids.
- Lipid biosynthesis in the rumen.
- Metabolism of phytal to phytanic acid.
Hydrolysis of esterified fat
The rumen bacteria produce two hydrolytic enzymes, cell bound esterases and lipases. The triglycerides undergo rapid hydrolysis and give rise to high concentration of unesterified fatty acids especially when concentrates are supplemented in the diet.
Biohydrogenation of unsaturated fatty acids
The bacteria are capable of synthesising long-chain fatty acids and phopholipids from short chain fatty acids.
The unsaturated fatty acids like linolenic acid present in grass is rapidly hydrogenated in the rumen yielding cis-trans dienoic acid, cis- trans monoenoic acids and stearic acid. This ability is present in bacteria as well as in protozoa. In a similar way linoleic acid is converted to conjugate dienoic acid, monoenoic acid and finally to stearic acid. The ability for biohydrogenation is greater with organisms like Butyrivibrio fibrisolvens and Ruminococcus albus. The hydrogen for biohydrogenation comes from the metabolic end products of carbohydrate fermentation.
Lipid biosynthesis of fatty acids
Acetate propionate and butyrate serve as the prime molecules for fatty acid synthesis and the malonate is used as extender of carbon chain in the fatty acid.
Metabolism of phytal to phytanic acid
The chlorophyll of the leaf contain the isoprenoid alcohol known as phytal.. The phytal is hydrogenated to dihydrophytal which then get oxidized to phytanic acid and incorporated in rumen organisms.
Synthesis of vitamins
Vitamin K and many B complex vitamins (thiamine, riboflavin, nicotinic acid, pantothenic acid, cyanocobalamine, pyridoxine and biotin) are synthesised in rumen are absorbed from the rumen. When a diet is deficient in cobalt, synthesis of vitamin B12 will be inadequate.
Detoxification of toxic substances
Toxic substances in plants (e.g. oxalates and gossypol) are detoxified in the rumen.
Absorption of fat from the Fore-Stomach
Absorption in the first three compartments is more rapid than in the abomasum.
The ruminal papillae increase the absorptive surface area. In ruminants, the short chain fatty acids are absorbed from rumen, reticulum, and omasum
The rate of absorption of acetic, propionic and butyric acids increases as the rumen pH gets decreased. The rate of absorption of butyric acid is > propionic > acetic acid.
Large amounts of lactic acid is produced during excessive feeding of cereals or foods rich in sugars. Lactic acid has a lower rate of absorption (1/10 of VFAs).
Part of the acetate is absorbed as such, whereas about 30% of the acetate are converted to lactate in the ruminal epithelium. All of the butyrate is converted to beta OH butyrate in the ruminal epithelial cells before absorption.
Propionate is absorbed unchanged. Short chain fatty acids and water are absorbed from omasum. Ammonia is also absorbed from rumen and its absorption is more rapid at pH 6.5.Sodium ions are absorbed against the concentration gradient and are actively absorbed.
Potassium ions are actively secreted in rumen. (Potassium ion is high in rumen compared to plasma and sodium concentration is less in rumen than plasma.)
Rumen wall is impermeable to phosphate, calcium and magnesium ion. Water is absorbed and hypotonic to plasma.